1. Field of the Invention
The invention relates to a method and system for transmitting data packets in time slots on a communications network having first and second unidirectional buses which are oppositely directed and a plurality of access units coupled between the buses, said method including the steps of forming a distributed queue of all of the data packets by establishing in each access unit a queue of the data packets therein which are to be transmitted on the first unidirectional bus, transmission by each access unit of a request flag on the second unidirectional bus for each data packet in the queue in such access unit, keeping count in each access unit of the number of request flags passing it on the second unidirectional bus, and transmitting the data packet at the head of the queue in an access unit when a predetermined number of empty time slots has passed such access unit on the first bus.
2. Description of the Related Art
A method of this type is described in the International Patent Publication No. WO 86/03639 published June 18, 1986. According to this prior art method a request flag for a data packet is transmitted on the second bus upon arrival of said data packet at the head of the queue in the access unit. Simultaneously the instantaneous request counter value is loaded in a countdown counter whereafter the request counter is resetted. Thereafter the request counter starts accumulating again the number of request flags passing on the second bus, whereas for each empty time slot detected on the first bus the access unit supplies a decrement signal to the countdown counter. The first empty time slot detected by the access unit after the countdown counter has reached a predetermined end value (e.g. zero) is used by the access unit to transmit the data packet on the first bus. If the queue in the access unit comprises at least one more data packet to be transmitted, the new instantaneous request counter value is loaded into the countdown counter and the whole procedure is repeated until the last data packet in the queue is transmitted. If there are no more data packets in the queue the request counter still accumulates the passing request flags, transmitted by downstream access units, but is simultaneously decremented by each empty time slot passing on the first bus in downstream direction. Therewith the access unit keeps track of all still pending and not answered request flags transmitted by downstream access units.
According to this method each data packet first has to travel through the queue in an access unit. As it reaches the head of the queue the data packet has to wait further for the the passing of a number of empty time slots requested by downstream access units before it can be transmitted. No consideration is given to the length of the queue in an access unit. That implies that the period between the time of arrival of a data packet in an access unit and the time it is actually transmitted will be significantly shorter for an access unit with comparatively low traffic than for an access unit with comparatively busy traffic. It is furthermore found in practice that the waiting period for data packets arriving in an access unit at a relatively short distance from the time slot generator is in average significantly shorter than the waiting time for data packets arriving at an access unit more distant from the time slot generator. This prior art method does not have any provision for assigning predetermined priority to each of the access units. In practice, however, a priority assignment is considered as very helpful in many cases, for instance to avoid relative by long waiting times in access units with high traffic density.